Shalom, all,
My two cents:
Having a common belief in Yeshua as Lord
does make two people spiritually sister and brother. But not all of my sisters in Yeshua would make good wives for me, nor would I make a good husband to them.
There are many practical considerations. Is the Messianic offended by a Christmas tree and Christmas celebration, thinking it pagan, or is s/he okay with it? Do they plan on observing Sabbath? Do they plan on attending a Congregation on Sabbath or Sunday?
Just being in Messiah doesn't automatically mean you can marry any other person in Messiah. Consider any other two Christians getting married: what about a Catholic and a Baptist? A Mennonite and Orthodox? A Pentecostal and a High-Church Episcopalian?
So, I would say that yes, they are equally yoked, but not necessarily are they going to get along.
In Messiah,
Daniel